The present invention relates to a process for the production of a diode array and liquid crystal display screen.
The attached FIG. 1 shows a known display screen constituted by an upper plate 10 covered by transparent conductive strips 12 oriented in accordance with parallel rows and a lower plate 20 covered with a block matrix 22 and transparent conductive strips 24 oriented along parallel columns. These two plates are assembled and kept spaced by a shim and a liquid crystal is introduced into the space separating them. Each block constitutes an elementary display zone or "points". A block is electrically connected to the adjacent column by a bridge 26. The exciting of a bridge Pij of the matrix is obtained by applying appropriate voltages to row Li of the upper plate and to column Cj of the lower plate.
There are several known embodiments of bridges 26. Certain of them use transistors (and in particular thin film transistors), whereas others use non-linear elements such as diodes, which makes it possible to increase the number of addressable rows and columns, i.e. the number of display points.
The present invetion relates to a process for producing such a screen and more particularly relaes to the production of bridges 26 in the variant using diodes.
The prior art essentially discloses three different diode bridge types:
1. In the article entitled "Integrated Matrix Address LCD Amorphous Silicon Back to Back Diodes" published by N. Szydlo et al in "Japan Display", 1983, pp. 416-418, a bridge is described, which comprises two back to back diodes. The bridge is formed by a n.sup.+ doped a-Si:H layer, a non-doped or slightly doped a-Si:H layer, an insluating layer with two openings and two metal (platinum) pads traversing the openings and forming Schottky contacts with the a-Si:H layer.
2. In the article entitled "A 210.times.228 Matrix LCD Controlled by Double Stage Diode Rings" published by Sergo Togashi et al in the journal . . . pp. 141-144 is described a bridge having two parallel diodes forming a ring. Each diode is of the PIN type and comprises an indium and tin oxide layer (ITO), a chromium layer, a p-doped a-Si, an intrinsic a:Si and a n-doped a:Si stack, a chromium layer, a SiO.sub.2 layer having an opening and an aluminium layer covering the complete assembly and in contact with the upper chromium layer.
3. In the article entitled "A 250.times.240 Element LCD Addressed by Lateral MIM" published by S. Morozumi et al in "Japan Display" 1983, pp. 404-407, a description is given of a bridge comprising a tantalum layer, a thin tantalum pentoxide deposit, a polyimide layer and a chromium layer (MIM or metal-insulator-metal structure).
Although satisfactory in certain respects, the above procedures lead to complex production processes. In particular the first two require four different photolithiography operations, i.e. four masks, with the inevitable alignment problems caused thereby. The third procedure only requires three photogravure operations, but requires a difficultly reproducible thin insulating film.